The invention relates to a process for the preparation of cyanoacetic acid esters of the, general formula 
starting from an alkoxypropionitrile.
The residue R is to be understood here and hereinafter as meaning an optionally substituted linear or branched C1-8-alkyl group or an aryl-C1-4-alkyl group. Unsubstituted C1-8-alkyl groups are, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, butyl, pentyl, hexyl, 2-ethylbutyl, heptyl, octyl or 2-ethylhexyl. 2-Ethoxyethyl or 2-methoxyethyl may, for example, be employed as substituted C1-8-alkyl groups. Aryl-C1-4-alkyl groups are, for example, benzyl, 1-phenylethyl and 2-phenylethyl.
Cyanoacetic acid esters of the general formula I are important starting materials for the synthesis of organic compounds, for example pharmaceutical active substances.
The electrochemical oxidation of alkyl xcex2-cyanoethyl ethers (alkoxypropionitrile""s) with platinum or lead oxide as anode in aqueous sulfuric acid is known (B. Wermeckes, F. Beck, Elektrochim. Acta. 1985, 30, 1491). In this reaction, the main products obtained are not the desired esters but cyanoacetic acid and the carboxylic acid corresponding to the alkyl group such as, for example, formic acid or acetic acid.
WO 92/01296 describes the oxidation of partially oxidized propionitriles (e.g. cyano-acetaldehyde or acetals thereof) by means of oxygen or other oxidants in the presence of catalysts such as iron chloride or palladium chloride, cyanoacetic acid likewise being formed first, which then has to be converted into a cyanoacetic acid ester by acid-catalysed esterification with the appropriate alcohol.
The object of the present invention is to make available a process for the preparation of cyanoacetic acid esters in which the desired cyanoacetic acid esters are formed directly.
This object is achieved by the process according to the invention.
Surprisingly, it has been found that if alkoxypropionitriles of the general formula 
wherein R has the meaning given above are oxidized using oxygen or an oxygen-forming reagent in the presence of a catalyst based on lead or one of the transition metals, the cyanoacetic acid esters of the general formula I are formed directly.
Depending on the meaning of the radical R and the ester to be produced, starting materials which can be employed are, for example, 3-methoxypropionitrile, 3-ethoxypropionitrile, 3-propoxypropionitrile, 3-butoxypropionitrile and 3-benzyloxypropionitrile. These starting materials are either commercially available compounds or can be prepared according to known syntheses (cf. e.g. B. Wermeckes, F. Beck, Elektrochim. Acta, 1985, 30, 1491), for example by addition of the corresponding alcohol to acrylonitrile.
An oxygen-forming reagent which can be used is, for example, hydrogen peroxide.
Transition metals are understood below as meaning, in particular, the metals platinum palladium, ruthenium, rhodium, rhenium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdenum, osmium, silver, cadmium, tantalum, tungsten or mercury.
A cobalt catalyst such as, for example, cobalt acetate tetrahydrate, or cobalt acetylacetonate, is preferably employed for the oxidation.
The cobalt catalyst most preferably employed is cobalt acetate tetrahydrate.
Preferably, the transition metal catalyst is employed in an amount from 0.01 to 10 mol %, more preferably in an amount from 0.01 to 3 mol %.
The oxidation is expediently carried out at a temperature of 50 to 250xc2x0 C., preferably at a temperature of 100 to 200xc2x0 C.
The oxidation is customarily carried out under pressure, such as, for example, a pressure of 5 to 15 bar.
The oxidation is carried out either without solvent or in an inorganic or organic solvent. The organic solvent employed can be, for example, acetonitrile, acetic acid, toluene, ethyl acetate, acetone or tetrahydrofuran, or an alcohol such as methanol, ethanol, propanol or butanol. The inorganic solvent used can be, for example, water. An organic solvent such as acetonitrile is preferably employed.
The oxidation is expediently carried out in the presence of a radical inducer such as, for example, N-hydroxyphthalimide, N-hydroxysuccinimide or N-hydroxymaleimide.